docs: add CONTRIBUTING.md (fixes broken link from README)

[danilaverbena]

Problem

The README's Get Involved section points contributors to a contributing guide:

| 🔧 **Submit a PR** | See [Contributing Guidelines](CONTRIBUTING.md) — start with [`good-first-issue`](...) |

but there is no CONTRIBUTING.md in the repository, so the link 404s. New contributors following it hit a dead end.

Change

Adds a CONTRIBUTING.md tailored to Setu rather than a generic template:

• Ways to contribute and a pointer to good-first-issue, aligned with the existing issue templates.
• Development environment: Rust 1.75+ (2021 edition), RocksDB, optional Docker; cargo build --release / cargo test --all, plus the scripts/ helpers for local multi-validator/e2e runs.
• Project layout: a short map of the Cargo workspace, linking back to the README's Project Structure.
• Workflow: fork → topic branch → cargo fmt / cargo clippy -D warnings / cargo test --all → Conventional Commits → PR against main linking the issue.
• ⚠️ Consensus-critical changes: an explicit warning not to change on-chain encodings/hashing/state or bump the pinned bcs = "=0.1.6" without a coordinated state migration (mirrors the note in the root Cargo.toml).
• PR checklist, security-reporting guidance (private disclosure via Discord), a neutral license note (points to the LICENSE file), and community links.

This fixes the broken README link and gives contributors a concrete starting point.

Notes

• The license section intentionally points to the LICENSE file rather than naming a license, to avoid asserting one. (Heads-up for maintainers: the README's License section currently says Apache License 2.0, while the committed LICENSE file is AGPL-3.0 — worth reconciling in a separate change.)
• Docs-only; no code changes.

Happy to adjust tone, sections, or specifics to match how the team prefers to work.